The processing of semiconductor wafers and other integrated circuits (IC) includes critical manufacturing steps such as etching wafer surfaces and depositing layers of material on wafer surfaces to form device components, interconnecting lines, dielectrics, insulating barriers and the like. Various systems have been employed to deposit layers of material and the like on the surface of integrated circuits, and often such layers are formed by chemical vapor deposition (CVD). A conventional thermal CVD process deposits a stable chemical compound on the surface of a wafer by thermal reaction of certain gaseous chemicals. Various CVD reactors have been used in the art including low pressure CVD systems and atmospheric pressure CVD systems.
More recently, plasma enhanced (sometimes called plasma assisted) CVD systems (PECVD) have been developed. PECVD systems generally operate by disassociation and ionization of gaseous chemicals. The high electron temperatures associated with the plasma increase the density of the disassociated species available for deposition on the water surface. Accordingly, such systems are able to operate at lower temperatures than conventional thermal CVD systems. Such lower temperature processes are desirable and minimize diffusion of shallow junctions and inter-diffusion of metals contained within the integrated circuits. Moreover, PECVD systems are suitable for forming multiple dielectric layers to be used to isolate stacked device features as device densities increase. When forming such multilayer dielectric layers it is desirable to provide a layer with good gap fill, isolation, stress and step coverage properties. These properties become more difficult to attain as device dimensions shrink.
To address the continual need for reactors capable of producing high quality wafers with increasing smaller device features, high density plasma (HDP) CVD (HDP-CVD) systems have been developed. HDP-CVD systems employ a plasma generating source capable of producing a high density of ions, typically on the order of 10.sup.11 plasma ions/cm.sup.3 and above. Various types of plasma generating sources have been reported in the prior art. One example of a plasma generating source is found in U.S. Pat. No. 4,918,031 which discloses an electrostatically shielded RF (ESRF) plasma generating source. The source has one longitudinally split, metallic shield disposed within a helical coil. An elaboration of the '031 patent is found in U.S. Pat. No. 5,234,529, which employs a plurality of longitudinal slots formed in the shield. The shields have been used in the art to suppress the amount of capacitive coupling between the plasma source (i.e. the RF coil and the like) and a plasma region, maximizing the inductive coupling to capacitive coupling ratio. The shield exposes only a small area of the plasma source to the plasma region, thus limiting substantially the capacitive component of the coupling of the electromagnetic energy into the plasma generating region. It is generally believed in the prior are teachings that capacitive coupling may increase the possibility of damage to the semiconductors being processed, and these teachings thus indicate the need for pure or near pure inductive coupled plasma.
It has bean found by the inventors that limiting the capacitive coupling through the shield creates a number of significant problems. First, ignition of the plasma generating source is very difficult and often times requires the use of supplementary igniters. For example, a spark coil, a low power RF bias shield, or even a low power start sequence using the wafer support bias power is needed to ignite the plasma generating source. The addition of such supplementary igniter components, as well as matching networks necessary for their power coupling, increases the complexity, cost and inefficiency of the system.
The '529 patent attempts to address the ignition problem by varying, the area of the slots by using two concentric cylindrical shields, disposed one behind the other, that move to vary the exposed area and increase the capacitive coupling. Such an arrangement is cumbersome and increases the mechanical complexity and cost of the source. Further the effect of varying the area of the slots can even lead to such detuning of the source that the matching network cannot correct the change of the source input impedance and the resonant frequency.
Another problem associated with plasma generating sources in general, is obtaining uniform distribution of the plasma across the wafer to be processed. Prior art systems typically employ supplemental magnetic sources at various points in the reactor to confine and distribute the plasma. For example, a dc coil placed below the plasma generating source and close to the wafer, or a system of two coils, or an array of permanent magnets may be used. Again, such arrangements increase the complexity and cost of the system. Moreover, the inventors have reason to believe that supplementary magnetic fields can actually have an adverse effect and cause a worsening of deposition uniformity.
A further issue with plasma generating sources is the ion density achievable by the source. There is continued interest in the pursuit of ever higher plasma densities to address the unyielding efforts to shrink device dimensions. The conventional plasma source as taught by the prior art with the electrostatic shield, always limits to some degree the ion density achievable by the source. To achieve greater ion densities, the inventors have found that it is desirable for the plasma generating source to have the effective radiating length of the slots in the shield extended into the high current region of the source. By doing so, the inductive coupling from the source into the plasma is greatly increased, while keeping the undesirable capacitive coupling unchanged.
Semiconductor processing requires that the plasma generating source be capable of operating over a wide pressure range. During processing of a wafer, the plasma source and reactor operate at very low pressures on the order of 10 mTorr and less. During cleaning of the reactor however, the source will operate at pressures of up to 1.0 Torr and above. Cleaning of the reactor plays an important role in the effective operation of a system. The highly reactive species deposit on the walls of the chamber, and the operating components, as well as on the surface of the substrate. Such deposits affect the operation of the system, may affect the plasma potentials within the system, and are a serious source of particulates which may end up contaminating the film deposited on the wafer. Accordingly it is advantageous that the plasma source operate over a wide pressure range.
Thus, it is desirable to provide a plasma generating source that addresses the foregoing problems; such as, a source that is capable of self-ignition, operates over a large pressure range, and achieves high ion density, while at the same time keeping the capacitively coupled component of the electromagnetic energy as low as possible and increasing the inductively coupled component. It would be particularly advantageous to provide a source that promotes uniform plasma distribution, and uniform qualities in the resultant deposited film.